Jaundice is the most common clinical diagnosis in the newborn, with 60% of neonates displaying significant unconjugated hyperbilirubinemia during the first 3 days of life. Phototherapy is the most common treatment, with hundreds of thousands of babies in the U.S. being irradiated each year with white or blue light to decrease plasma levels of unconjugated bilirubin and thus reduce the risk of permanent brain damage. Yet the incidence of jaundice is increasing, following early discharge from hospitals, and kernicterus has re-emerged after having largely disappeared. The broad, long-term objectives of the project are to understand the interrelationship between bilirubin 3-D structure, physico-chemical properties and hepatic processing, and improve the efficacy of phototherapy. The specific aims are to: 1) Fully characterize the 3-dimensional structure of bilirubin, its anions, conjugates and photoisomers in biomemtic/physiologic environments using molecular dynamics computations and sophisticated spectroscopic methods. 2) Clarify the carboxylic acid pKa of bilirubin, its dependence on hydrogen bonding and aggregation, its influence on polarity and its role in bilirubin transport and metabolism. Is bilirubin an ionophore and ion transporter? 3) Investigate the role of bilirubin phototautomerization and photodissociation, elucidate the mechanism of photocyclization to lumirubins and its stereochemistry, and promote higher yields of E-photoisomers in order to improve the efficacy of phototherapy; explicate the influence of local environment including protein binding, on the channeling of bilirubin photochemical reaction pathways. 4) Synthesize chemically-designed bilirubins of well-defined and varied secondary structure, with differing intramolecular hydrogen bonding and amphiphilicity and substituents of varying size and polarity to: (a) define the physico-chemical properties important in metabolism and phototherapy, and (b) use as biologic molecular probes of the interrelated roles of pigment stereochemistry and intramolecular hydrogen bonding, polarity and pKa acidity, vinyl/C(10) substituents and perimeter hydrophobicity in hepatic uptake, conjugation and efflux. 5) Design and synthesize bilirnbin fluorophores and MRI-sensitive imaging agents and Trojan Horse vehicles for research and clinical applications involving pigment distribution, hepatic uptake, clearance and drug delivery. These studies are directly relevant to the prevention of neurological damage in the jaundiced newborn, to improving our understanding of bilirubin detoxification and liver disease. They are designed to clarify how the molecular structure of bilirubin controls its transhepatic transport, intracellular partitioning, glucuronidation and ABC-cassette MRP2-assisted canalicular secretion, and to lead to improved and safer methods of treatment for severe unconjugated hyperbilirubinemia in the neonate and in patients with Crigler-Najjar syndrome. [unreadable] [unreadable]